1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system.
2. Description of the Related Art
In recent years, imaging apparatuses including a flat panel detector (hereinafter abbreviated as “FPD”) formed of a semiconductor material, have started to be put to practical use as imaging apparatuses that used for medical diagnostic imaging or a nondestructive inspection using an X-ray. In an FPD, a plurality of pixels having conversion elements constructed using semiconductor material such as a-Si that is capable of converting radiation into electric charges and switching elements that transfer electric signals that correspond to the electric charges are arranged two-dimensionally. Such imaging apparatuses including an FPD are used, for example, in medical diagnostic imaging as digital imaging apparatuses for still image radiographing like general radiographing or moving image radiographing such as fluoroscopic radiographing.
When performing radiographing, a radiation imaging apparatus performs radiographing in synchrony with the operations of an X-ray generating apparatus. As synchronization methods, for example, a unit is available in which the X-ray generating apparatus and the radiation imaging apparatus are synchronized electrically connecting the two apparatuses to each other, or a unit is available in which the radiation imaging apparatus is synchronized with the X-ray generating apparatus by detecting X-ray that is radiated from the X-ray generating apparatus. In the former case, because service personnel connect the X-ray generating apparatus and the radiation imaging apparatus with a cable, the connection work involves time and labor, and furthermore the X-ray generating apparatus and the radiation imaging apparatus must be fixed and used as a single pair of apparatuses. In the latter case, a method is known in which an X-ray detector is provided inside and outside the radiation imaging apparatus, or in which the radiation imaging apparatus itself performs detection of radiation, and in this case there is the advantage that time and labor for connection work is not required and the radiation imaging apparatus is portable and can be used in combination with various X-ray generating apparatuses.
Normally, in an FPD, pixels that include a photoelectric conversion element and a switching element are two-dimensionally arrayed, and reading of signals from the photoelectric conversion elements and resetting of the photoelectric conversion elements is performed in row units. Before X-ray is irradiated, the switching elements are subjected to on/off control in row units, and a dark current component flowing to the photoelectric conversion elements is reset (“initializing operation”). If an X-ray irradiation signal is input or X-ray is detected during an initializing operation, it is necessary to immediately end the reset operation and transition to an accumulation operation. If a transition is not made to the accumulation operation even though an X-ray irradiation signal has been input, a time lag will arise between the time that the user pushed the exposure button and the actual photographed image, and an unintended image in which a difference in level or the like arises will be obtained. Further, if an initializing operation is continued even though X-ray was detected, since X-ray signals that are generated at the photoelectric conversion elements will be reset, unnecessary X-ray will have been irradiated at the subject and the amount of radiation exposure may increase.
Japanese Patent Application Laid-Open. No. H11-151233 discloses technology that includes a radiation detection unit and that immediately shifts the operating state from a radiographing preparation state an accumulation state when the start of radiation irradiation is determined. Further, Japanese Patent Application Laid-Open No. 2010-268171 discloses a radiation image radiographing apparatus that detects a current that flows through a bias line that supplies a bias voltage to a radiation detecting element, detects the start irradiation of radiation based on a value of the current that is detected, and holds an electric charge that is generated inside the radiation detecting element.
However, in Japanese Patent Application Laid-Open No. H11-151233 and Japanese Patent Application Laid-Open No. 2010-268171, in some cases it is not possible for the radiation detection unit to adapt to various radiographing kinds (the radiographing site and the build of a subject, a moving image or a still image radiographing mode, and the like). In diagnostic imaging using X-ray, X-ray irradiation conditions differ depending on the kind of radiographing. Therefore, a radiation detection unit is demanded that is capable of accurately detecting irradiation of X-ray with respect to various irradiation conditions.